The proposed project aims to analyze various aspects of the morphology and physiology of mammalian extraocular muscles (EOM). The morphological study is based on the light and electronmicroscopic examination of transverse sections of muscles and it is in preliminary stage. We aim: a) to recognize the different cellular populations of rat EOM and their distribution within the global and orbital layers; b) to verify whether morphological variability is present or not along the fiber length; c) to identify fibers which have been labelled with dye spots after being studied electrophysiologically. The physiological study is carried out using the "in vitro" preparation of the inferior rectus muscle of the rat, which we developed previously. Two aspects are being studied: the electrophysiological properties of muscle fibers and the energetic requirements of tonic fiber contractility. The first aspect is examined by using conventional technique for intracellular recording. Previously, we have demonstrated that the two main populations of fibers in the global layer of the muscle have clearly different electrical properties. The study now in progress involves the analysis of muscle fibers in the orbital layer. There are morphological indications suggesting that orbital fibers may have different than global fibers. The magnitude of input resistance and time constant, the ability to give action potentials, and the spontaneous evoked synaptic activity will be examined. Orbital fibers will be labelled with a dye after electrical recording for further identification at the light and electronmicroscopic levels. The morphology and distribution of nerve endings will be studied using cholinesterase staining. Previous work has demonstrated that the tension produced by tonic fibers is rapidly reduced by lowering the oxygen tension in the bathing fluid and slowly depressed by glucose removal. More recent experiments have demonstrated that the inhibition of ATP synthesis with dinitrophenol reduces markedly and in a few minutes the tension produced by tonic fibers. The energetic requirements of contractility will be examined by using other metabolic inhibitors, especially to block the synthesis of phosphocreatine, by determining in a quantitative manner the relationship between contractility and 02 pressure, and by studying the effect of temperature.